Louisiana's outer continental shelf is the most extensively developed region in the United States, and over the last two decades, drilling activities have increased. This area is also home to some of the most sensitive ecosystems in the world. The coastal wetlands and the Gulf contribute ˜28% of the total volume of U.S. fisheries and is also a rich habitat for several rare species. The wetland ecosystems buffer the landmass from hurricanes and tropical storms. The Mississippi river drains one-third of the United States and delivers the largest amount of sediment to the continental shelf.
Approximately 3 million gallons of crude oil are spilled into U.S. marine waters annually (National Research Counsel. Committee on understanding oil spill dispersants: efficacy and effects. Washington, D.C.; The National Acadamies Press, 2005, p. xviii, 377). While trends in volume and frequency of accidental releases have been decreasing for several decades, large spills, such as the recent massive oil spill in the Gulf of Mexico from the Macondo/Deepwater Horizon, occur intermittently (Mukherjee and Wrenn, Effects of Physical properties and dispersion conditions on the chemical dispersion of crude oil. Envirn Engin Sci. April 2011, 28(4): 263-273). Pollution of seawater by oil, be it crude oil or fractions of crude oil, occasioned by accidents, off-shore drilling operations, and/or discharge of ballast or spillage from oil tankers, results in the formation of a continuous film or slick of oil which tends to continuously spread. When these large spills occur, environmental and economic effects, including contamination of animals and shorelines, can be devastating. Oils spilled at sea lead to processes that alter the chemical properties of the oil. Once oil enters the sea it moves on the water's surface by advection and spreading. This movement increases the exposure area of the oil to subsequent weathering processes, one of which is emulsification. The dispersion of oil droplets in water (O/W emulsions), including natural dispersants in its formulation, contributes to a removal of the oil from the sea surface into the water column for rapid biodegradation.
The blowout from the Deepwater Horizon spill produced a gas/oil mixture and was located ˜1,500 m below the sea surface, where the specific conditions are high pressure (˜160 atm) and low temperature (48° C.). The oil reservoir was 13,000 ft (4,000 m) below sea bottom. The Flow Rate Technical Group convened to study the amount and the flow rate of the oil released has estimated that about 4.7-5.5 million barrels (1 bbl=160 L) were released and the flow rates range from 53,000 to 62,000 barrels per day. Oil dispersants were injected into the gushing plume at a volume ratio of ˜1% (˜0.8-0.9 L per 100 L oil) to disperse the oil/gas mixture with the goal of minimizing the overall environmental impact, partly to prevent oil from reaching the coastline.
Oil dispersant compositions are applied on the oil slicks, generally by spraying. Dispersants, typically some amphiphilic surfactant, promote the formation of oil droplets that can be dispersed into the water column (Clayton et al., Oil spill dispersants: mechanisms of action and laboratory tests. CRC Press, Inc. 1993, Boca Raton, Fla.; Mukherjee and Wrenn. Effects of Physical properties and dispersion conditions on the chemical dispersion of crude oil. Envirn Engin Sci. April 2011, 28(4): 263-273) by reducing the interfacial tension by accumulating at the oil-water interface making it easier for normal ocean movement to disperse the oil and interacting with both the oil and water phases simultaneously, allowing oil droplets to be transported vertically and horizontally in the water column thereby diluting the oil in the seawater. The primary constituents of chemical dispersants include low molecular weight surfactants and solvent. For instance, the formulation of Corexit® 9550 contains petroleum distillates, low molecular weight alcohols (which are the surfactants), and sulfonic acid salts (Corexit® 9550 MSDS, Nalco Energy Services, L.P., Jun. 14, 2005). Corexit® 9500, which was used to address the oil leakage from the Deepwater Horizon, contains about 48% nonionic surfactants, including ethoxylated sorbitan mono- and tri-oleates and sorbitan monooleate, and 35% anionic surfactants, including sodium dioctyl sulfosuccinate, in a solvent consisting of a mixture of food-grade aliphatic hydrocarbons (National Research Counsel. Committee on understanding oil spill dispersants: efficacy and effects. Washington, D.C.; The National Acadamies Press, 2005, p. xviii, 377; Pollino and Holdway, Toxicity testing of crude oil and related compounds using early life stages of the crimson-spotted rainbowfish (Melanotaenia fluviatilis). Ecotoxicol Environ Saf. 2002; 52, 180) and contains 2-butoxyethanol, which is toxic and was identified as a causal agent in the health problems experienced by cleanup workers after the 1989 Exxon Valdez oil spill (Elana Schor “Ingredients of Controversial Dispersants Used on Gulf Spill Are Secrets No More”. Energy and Environment, NY Times (Jun. 9, 2010)).
The oil dispersants disintegrate the cohesive oily film into small droplets and disperse the droplets into the water column, thereby breaking the film and permitting the transfer of light and air from the atmosphere. Petroleum distillates are necessary to facilitate penetration of the surfactants into the oil, wherein the surfactants ultimately form micelles with the help of wave energy. During micelle formation, oil is trapped in the interior of the micelle, effectively forming small droplets that pinch off of the larger oil slick. Electrostatic double layer forces and/or steric forces between the micelles minimize re-coalescence. To achieve an efficient dispersion, oil droplets must be less than roughly 100 μm. Even smaller droplets are preferred in order to increase surface area to volume ratio enhances biodegradation.
However, the relationship between the design of dispersants and effectiveness in oil spills is not well understood. Several factors play a role in effective dispersion, and these factors are not well understood (“The Use of Chemical Dispersants to Treat Oil Spills” Technical bulletin No. 4: International Tanker Oil Pollution Federation Ltd., 2005). Studies have shown that the physical and chemical properties of the oil, composition of the dispersant, mixing energy, mixing time, fluid dynamics, temperature, and salinity are all important factors that affect dispersant performance (National Research Counsel. Committee on understanding oil spill dispersants: efficacy and effects. Washington, D.C.; The National Acadamies Press, 2005, p. xviii, 377; Mukherjee and Wrenn. Effects of Physical properties and dispersion conditions on the chemical dispersion of crude oil. Envirn Engin Sci. April 2011, 28(4): 263-273). First, there must be a minimum wave energy. For instance, dispersants in the Exxon Valdez spill were ineffective due to a relatively calm waters; enough shear was not present to disperse the oil. Too much wave energy, may also be deleterious to effective dispersion. The properties of the oil are also important. More viscous oil is generally resistant to dispersion. This is especially important in that the viscosity of oil rapidly increases after a spill, as lower weight components quickly evaporate. Moreover, oil in water emulsifications, which sometimes form in spills, also restrict the ability of surfactant to reach the oil/water interface. Finally, the properties of the dispersant are important. The majority of formulations are proprietary, and so it is difficult to make intelligent decisions on a particular dispersant, other than through empirical laboratory tests.
Corexit is most effective on oil slicks, where the dispersant comes into direct contact with the oil. However, at the Deepwater Horizon spill site, the dispersant was introduced at the exit of the broken pipe, where oil is travelling at several hundreds of miles an hour, and likely broke the oil into small particles. Further, the dispersion efficiency of Corexit® 9500 on large, underwater oil leaks with high flow rates is unknown. It is uncertain if the dispersant is effective under such conditions, which may lead to semi-stable emulsions that contribute to the large underwater oil plumes. Moreover, Corexit® 9550 contains 2-butoxyethanol, which is toxic and was identified as a causal agent in the health problems experienced by cleanup workers after the 1989 Exxon Valdez oil spill (Elana Schor “Ingredients of Controversial Dispersants Used on Gulf Spill Are Secrets No More”. Energy and Environment, NY Times (Jun. 9, 2010)). Laboratory experiments showed that dispersants increased toxic hydrocarbon levels in fish by a factor of up to 100 and may kill fish eggs (Region IV Regional Response Team Dispersant Use Policy; Oct. 8, 1996).
Depending on their magnitude and location, irreparable damage may be inflicted on marine and coastal ecosystems. This oily film causes a barrier, preventing the transfer of light and air from the atmosphere into the seawater, and coats maritime life in a detrimental oil slick. One aspect to the Deep Horizon spill is that a significant amount of the hydrocarbon mass does not emerge on the sea surface but remains in the water column. As droplets approach the continental shelf, they may interact with subsea sediments, animals, and vegetation. The long-term impact of oil on the ecosystem, including oxygen depletion, oil contact with biota, such as finfish and shellfish, acute fish toxicity, vegetation oiling, dispersion and advection by wind and water onto beaches, wetlands, bays, harbors, estuaries, is unknown (Thibodeaux, et al. Marine oil fate: Knowledge gaps, basic research, and developmental needs; a perspective based on the Deepwater Horizon Spill. Envirn Engin Sci. February 2011, 28(2): 87-93). Once the oil reaches the bed sediment either at depth, in the marshlands, or on the beaches, its transport is one of multiphase systems traveling through or lodging in porous media. The transported oil will influence—even cause—transport or deposition/aggregation of bacteria and other (bio) particulates. These effects are particularly poignant in deep water, where mobility, biological uptake, and degradation processes are very slow (Thibodeaux, et al. Marine oil fate: Knowledge gaps, basic research, and developmental needs; a perspective based on the Deepwater Horizon Spill. Envirn Engin Sci. February 2011, 28(2): 87-93). Moreover, laboratory experiments showed that dispersants increased toxic hydrocarbon levels in fish by a factor of up to 100 and may kill fish eggs (Region IV Regional Response Team Dispersant Use Policy; Oct. 8, 1996).
However, as evidenced above, there is a continuing need to develop and improve spill response technologies. Although environmental damage cannot be completely prevented when accidental releases of petroleum occur, it may be possible to minimize the damage if a variety of complementary response alternatives are available. Accordingly, a composition of plant mucilage is useful for sequestering oil and trapping it for collection and dispersing the oil.